Several filters are needed in the radio-frequency part of a bi-directional radio apparatus such as a mobile station. Extra frequency components produced by a mixer as well as extra frequency components produced by a power amplifier have to be removed in the transmit branch. In the receive branch, filters are needed in order to achieve basic selectivity, protect a low-noise pre-amplifier, and to attenuate noise generated by the transmitter on the receive band. In the case of different transmit and receive frequencies a duplex filter is generally used to mutually separate the different directions of transmission. An antenna switch is used in systems in which the transmit and receive frequencies are the same, and in systems where transmission and reception take place both at different frequencies and at different moments of time. Other functional units in a radio-frequency front end include the aforementioned amplifiers, a directional coupler for measuring the transmission power for power control, and mixers.
Integration of successive radio-frequency units is difficult mainly because of the relatively large size of the filters. If, for example, an antenna switch, a low-noise amplifier (LNA) and a filter between them are integrated on one chip, the large size of the filter calls for relatively large connection strips that produce electrical stray quantities and inductive couplings which degrade the selectivity of the filter. Complete integration of a filter between active RF units with other units is therefore impractical.
Another thing that makes integration difficult is the fact that commercial components usually have input and output impedances of 50 Ω in order to make modular design easier. However, advantageous values for RF circuit input and output impedances are often different: for example, the optimum input impedance level of a LNA is about 100 to 200 Ω. If the amplifier has this input impedance, the matching to the standard impedance of the preceding circuit requires a separate matching circuit. This will increase both the size and cost of the radio apparatus. Moreover, the matching circuit causes additional losses on the signal on the transmission path, which, in turn, means a shorter talk time, among other things.
From the prior art it is known numerous structures aimed at achieving as high degree of integration of RF circuits as possible. Radios according to the prior art usually comprise at least one integrated component and discrete filters connected to it/them.
Patent document WO 93/14573 discloses a solution applicable to time division multiple access wherein all active components of the transceiver are integrated into a single circuit. A disadvantage of this solution is that there are matching problems between the integrated circuit and the filters external to it. In addition, the integrated circuit does not contain a directional coupler. An external directional coupler built into a printed circuit board is susceptible to electric disturbances, requires a considerable amount of space on the printed circuit board and, in addition, causes an extra loss of at least 0.5 dB in the transmitter chain, which has a direct impact on the current consumption of the communications apparatus.
From U.S. Pat. No. 4,792,939 a solution is known in which a duplexer, transmitter and receiver are integrated on one chip. In that solution the duplexer, a bandpass filter and a mixer are implemented using surface acoustic wave (SAW) technology. A drawback of the arrangement is that the matching circuits required by the SAW circuits are so large and the SAW circuits themselves are so lossy and have such a poor power capacity compared to the transmission power that application of the solution in a modern mobile phone is impossible.
U.S. Pat. No. 5,432,489 discloses a solution that uses transmission lines belonging to circuits of the transmitter branch bandpass filter or to matching circuit, as part of a directional coupler. This way, the directional coupler can be moved from the printed circuit board onto a low-loss substrate and inside the protective housing of the high-frequency filter. The advantage of the solution is that it saves space and reduces susceptibility to interference as well as the transmission loss caused by the directional coupler, but the disadvantage is that in other respects the integration problems remain.
From U.S. Pat. No. 5,903,820 it is known a solution in which an antenna filter AFI, antenna switch ASW, directional coupler DCO, low-noise amplifier LNA, mixers MIX, and a power amplifier PA are integrated into one entity. This entity forms one component on the printed circuit board of a mobile station. FIG. 1a shows a block diagram of said entity 10, which is to be integrated. FIG. 1b shows an example of the practical implementation of the circuit 10. In the example, all parts are assembled onto one and the same low-loss substrate S. The most space-consuming parts are the coaxial resonators 11 and 12 that form the most significant part of the antenna filter AFI. The parts are located inside a common housing SH that protects them from interference.
An advantage of the structure according to FIG. 1 is that the number of structural elements needed for matching at the input of amplifier LNA and output of amplifier PA is smaller as there is no need to provide matching to the 50 Ω impedance level. Additional advantages include a reduction of parasitic effects, reduction of the number of components inserted onto the printed circuit board of the communications apparatus and reduction of the area needed on the circuit board. A drawback of the structure is that the transmit branch bandpass filter 20 and receive branch bandpass filter 30, shown in FIG. 1a, are still separate units on the circuit board. The antenna, too, is a discrete component.
An object of the invention is to reduce the above-described disadvantages of the prior art. The structure according to the invention is characterized by what is expressed in the independent claim. Preferred embodiments of the invention are disclosed in the other claims.